The delivery of polynucleotides and other substantially cell membrane impermeable compounds into a living cell is highly restricted by the complex membrane system of the cell. Drugs used in antisense, RNAi, and gene therapies are relatively large hydrophilic polymers and are frequently highly negatively charged. Both of these physical characteristics preclude their direct diffusion across the cell membrane. For this reason, the major barrier to polynucleotide delivery is the delivery of the polynucleotide across a cell membrane to the cell cytoplasm or nucleus.
One means that has been used to deliver small nucleic acid in vivo has been to attach the nucleic acid to either a small targeting molecule or a lipid or sterol. While some delivery and activity has been observed with these conjugates, the nucleic acid dose required with these methods has been prohibitively large for practical application.
Numerous transfection reagents have been developed that achieve reasonably efficient delivery of polynucleotides to cells in vitro. However, in vivo delivery of polynucleotides using these same transfection reagents is complicated and rendered ineffective by in vivo toxicity, serum interactions, and poor targeting. Transfection reagents that work well in vitro, cationic polymers and lipids, typically form large electrostatic particles and destabilize cell membranes. The positive charge of in vitro transfection reagents facilitates association with nucleic acid via charge-charge (electrostatic) interactions thus forming the nucleic acid/transfection reagent complex. Positive charge is also beneficial for nonspecific binding of the vehicle to the cell and for membrane fusion, destabilization, or disruption. Destabilization of membranes facilitates delivery of the substantially cell membrane impermeable polynucleotide across a cell membrane. While these properties facilitate nucleic acid transfer in vitro, they cause toxicity and ineffective targeting in vivo. Cationic charge results in interaction with serum components, which causes destabilization of the polynucleotide-transfection reagent interaction and poor bioavailability and targeting. Membrane activity of transfection reagents, which can be effective in vitro, often leads to toxicity in vivo.
For in vivo delivery, the vehicle (nucleic acid and associated delivery agent) should be small, less than 100 nm in diameter, and preferably less than 50 nm. Even smaller complexes, less than 20 nm or less than 10 nm would be more useful yet. Delivery vehicles larger than 100 nm have very little access to cells other than blood vessel cells in vivo. Complexes formed by electrostatic interactions tend to aggregate or fall apart when exposed to physiological salt concentrations or serum components. Further, cationic charge on in vivo delivery vehicles leads to adverse serum interactions and therefore poor bioavailability. Interestingly, high negative charge can also inhibit in vivo delivery by interfering with interactions necessary for targeting. Thus, near neutral vehicles are desired for in vivo distribution and targeting. Without careful regulation, membrane disruption or destabilization activities are toxic when used in vivo. Balancing vehicle toxicity with nucleic acid delivery is more easily attained in vitro than in vivo.
Rozema et al., in U.S. Patent Publication 20040162260 demonstrated a means to reversibly regulate membrane disruptive activity of a membrane active polyamine by reversible conversion of primary amines to pairs of carboxyl groups (β carboxyl and γ carboxyl of 2-propionic-3-methylmaleic anhydride). Rozema et al. (Bioconjugate Chem. 2003, 14, 51-57) reported that the β carboxyl did not exhibit a full apparent negative charge and by itself was not able to inhibit membrane activity. The addition of the γ carboxyl group was reported to be necessary for effective membrane activity inhibition. However, because the vehicle was highly negatively charged, with both the nucleic acid and the modified polymer having high negative charge density, this system was not efficient for in vivo delivery.
By substituting neutral hydrophilic targeting (galactose) and steric stabilizing (PEG) groups for the γ carboxyl of 2-propionic-3-methylmaleic anhydride, Rozema et al. were able to retain overall water solubility and reversible inhibition of membrane activity while incorporating effective in vivo hepatocyte cell targeting (U.S. Patent Publication 20080152661).
We now describe new membrane active polymers and compositions made with the described polymers for use in delivery of nucleic acids to cells in vivo. These new polymers provide improved therapeutic potential over those previously described.